Recently trawl fishing has almost changed from the bottom water trawl fishing to the mid-water trawl fishing.
In the case of the mid-water trawl fishing, it is relatively simple to drag or tow a trawl net while completely maintaining it in spaced apart relationship with respect to the bottom of the sea, but it is extremely difficult to drag or tow the trawl net along the bottom of the sea when the ocean floor has ridges or projections and valleys or recesses.
Especially when it is discovered that a ridge or projection exists at the bottom of the sea in the forward direction in which the towlines are dragged or towed, the speed of a trawler is increased or the towlines or warps are wound by the winches, thereby causing the trawl net to raise away from the ridge or projection.
However, when the trawl net is raised away from the ridge or projection by increasing the speed of the trawler, all the fishing gear; that is, the trawl net and otter boards must be raised to float, but in this case, because of a high degree of inertia of the fishing gear, there arises the problem that it takes a long period of time to raise the fishing gear and, consequently, the response speed or time is not satisfactory.
In the case of winding the towlines or warps by the winches, the load acting on the main engine of the trawler is increased so that the speed of the trawler changes and consequently the depth of the trawl net varies. It follows therefore that the variation in depth of the trawl net must be taken into consideration when the length of the towlines or warps are being decided, otherwise is extremely difficult to raise the trawl net to a suitable depth.
In order to solve the above and other problems, Japanese Patent Publication No. 5015/1971 discloses a method, but according to this method, since the volume of each float is predetermined, the trawl net cannot be raised to a suitable depth. In addition, a large number of floats are needed to float the trawl net so that the above-described method has not been satisfactorily used yet in practice.